This can happen when using RemoteNodeStore and/or LocalNodeStore. It seems like the backend code (of LccTools and python-openlcb) is re-announcing remote nodes, at least in the case where the node ID is generated (nextInternallyAssignedNodeID)--other cases are not confirmed. It is not confirmed in python-openlcb, but it is confirmed in LccTools and pre-release (0.x) versions of TCS Throttle App.